Study of laser-induced breakdown in a 30-cm air gap under a uniform field

In an ongoing effort to understand how ultrashort pulse lasers can be used to trigger spark discharges in air over considerable distances at electric fields much below that of self-breakdown, we have studied the influence of the laser pulse energy on the development of an electrical discharge over a 30-cm air gap in a uniform field geometry, In addition to the purely electrostatic effects associated with the production of free electrons as the laser beam is focused in the gap, the results highlight the importance of thermal effects to trigger the discharge when the plasma is produced several tens of microseconds before the electric field is applied. One of the most important mechanisms leading to breakdown of the gap is the production of a space leader from the laser-created plasma. Results showing the propagation of such a space leader in a 30-cm gap are presented for the first time.